1. Field of the Invention
The present invention relates to a photopolymerizable composition and, more particularly, to a photopolymerizable composition useful as a liquid photoimageable solder mask which is superior in ultraviolet rays curability and solder heat resistance, said composition being able to form images by the steps of ultraviolet rays exposure, after film-forming, and development by a diluted alkali aqueous solution.
2. Description of the Related Art
Printed wiring boards are often used to mount electronic devices compactly. The printed wiring board is produced by etching a copper foil bonded onto a laminate according to the circuit wiring and is soldered after arrangement of the electronic devices at a predetermined position.
A solder mask is usually applied before soldering the electronic devices to the printed wiring board and is coated to form a film in all areas except the areas where circuit conductors are soldered. This film is indispensable because it functions not only as an insulating membrane preventing the solder from adhering to unnecessary areas during the soldering, but also as a protective membrane preventing the circuit conductor from being corroded by oxidation or moisture after exposure to the air.
Hitherto, the solder mask have been formed on the board by screen-printing same and curing the print by ultraviolet rays or heat. However, not only has the printed board been steadily refined, enlarged and made into one-board, thus becoming more advanced with remarkable tempo, but also its mounting methods have evolved into a surface mount technology (SMT), and therefore the solder mask is also required to be of higher resolution, accuracy and reliability to accompany the refined SMT. Various improvements in the solder mask .have been proposed from a screen printing method to a liquid photoimageable solder mask method, which are superior in the positional accuracy and the covering power on the conductor edge regardless of the boards of conventional or professional use. For example, a solder mask composition comprised of bis-phenol type epoxy acrylate, sensitizer, epoxy compound, epoxy curing agent is disclosed in Japanese Patent Provisional Publication No. 50-144431 and Japanese Patent Publication No. 51-40451. In the case of these solder masks, the unexposed areas are removed and developed by an organic solvent. However, there are the risks of environmental pollution, fire and so on since a large amount of the organic solvent is used in the removal (development) of the unexposed areas. In particular, regarding the problem of environmental pollution the influences on the human body have been given a great deal of attention recently and it is the real state that we attempt with difficulty to seek for any countermeasures against this problem.
In order to solve this problem, an alkali developing type photoimageable solder mask capable of development by a dilute alkali aqueous solution is proposed.
As for an ultraviolet curing agent capable of alkali development, a material consisting of a reaction product as a base polymer, which is obtained by reacting epoxy resin with unsaturated monocarboxylic acid followed by adding polybasic acid anhydride, is disclosed in Japanese Patent Publication Nos. 56-40329 and 57-45785. In addition, a liquid solder mask composition using a novolak-type epoxy resin, which is superior in heat and chemical resistance, can be developed by a dilute alkali aqueous solution and is disclosed in Japanese Patent Previsional Publication No. 61-243869.
However, the above mentioned solder mask compositions have problems in that after application they induce developing failure because of the advancement of the heat curing at the time of drying, and they cause electrolytic corrosion and discoloration of the surface of copper foil, depending on the combinations of polybasic acid anhydride, which is employed in order to make the mask soluble in alkali developer, epoxy resin and epoxy resin curing agent. The compositions also have the problem of deteriorating the electrical characteristics of the board due to the influence of the carboxylic acid produced by the reaction of polybasic acid anhydride. In addition, their usage is restricted because deterioration of heat resistance and adhesion are induced when the mixing rate of epoxy resin, i.e., the heat curing component used to obtain the features as a solder mask, is small, and difficulty in developing in the dilute alkali solution is induced when the mixing rate of epoxy resin is large. Furthermore, there are additional problems with the above mentioned solder mask compositions in that they take a long exposure time because of the slowness of the curing by ultraviolet rays and sufficient heat resistance has still not been obtained.